The IPCC - the Independent Police Complaints Commission - is investigating claims from a mum that police used unnecessary force on her while she was pregnant.
Lynnette Wallace, 41, was seven and a half months pregnant at the time of her arrest, and went into premature labour three days later.
She had been picked up by police in July last year on suspicion of blowing up a car in Nottinghamshire. She was charged with witness intimidation.
While in custody, Miss Wallace told the police she had a history of self-harm. She claims they told her to remove a cardigan she had draped around her shoulders and when she refused, alleges they used unnecessary force on her.
"They came back in and wanted me to give them my cardigan but I refused because I only had a thin top on underneath," she said.
Miss Wallace further claims she was treated with too much force by Nottinghamshire Police officers the following day, and shortly after began to experience stomach pains.
After being remanded at Peterborough Prison by magistrates she began bleeding and was taken to hospital where she had an emergency caesarean.
Her little girl Charna - her seventh baby - was born ten weeks early, weighing just 3lb, 3oz.
Miss Wallace says that while she was in custody she missed a medical appointment for the result of breast cancer screening and later found out she did have cancer.
She said: "At one point, I wasn't sure if she was going to live or die. It was a nightmare, there is no other way the describe it from the birth to finding out I had breast cancer a few weeks later.
"I have never gone through so much stress in my life."
Witness intimidation charges against Miss Wallace were later dropped, but she spent five weeks in prison after the birth of her baby, who was kept in hospital for the duration.
The Independent Police Complaints Commission has confirmed it has started an investigation into Miss Wallace's claims. A spokesperson said: "The IPCC can confirm it is independently investigating the treatment of a woman while she was in Nottinghamshire Police custody in July 2011. This investigation is ongoing. Several officers have been served with notices advising them that their actions are subject to investigation."
A police spokesperson said: "Nottinghamshire Police can confirm a public complaint from a woman is being investigated independently by the IPCC over a claim of unnecessary force.
"No officers have been suspended and any information required by the IPCC will be provided."